Sol-gel processing is a common method for the production of glass or ceramic materials. It involves the transformation of a liquid or colloidal sol into a gel which upon curing removes the liquid phase from the gel to provide a solid material. Typically a sol-gel precursor is subjected to a series of hydrolysis and condensation reactions to form a colloidal suspension which subsequently condenses into a gel network. The condensation occurs with the loss of a by-product that is ultimately driven from the gel to form the solid material. The process permits the formation of powders, monoliths, fibers, membranes, aerogels, and films.
The first and still one of the largest applications for sol-gel technology is for the formation of thin films, generally 0.5 μm or thinner. The films are generally produced on substrates that have been coated with the sol, the sol generally applied via spraying, electrophoresis, inkjet printing, roll-coating, dip-coating or spin-coating. The resulting coating constitutes a protective, decorative or optical coating. Optical coatings that are reflective or antireflective have been formed via sol-gel processing.
Coatings for optical applications (e.g. infrared and visible light) can be prepared via sol-gel techniques. Traditional sol gel processes generally involve the addition of water as a reagent to form the sol, often in excess of other constituent used. The resulting sol-gel materials generally have a high affinity for water. However, coatings for certain infrared applications should be substantially free of water or molecules comprising hydroxyl (—OH) groups to avoid absorption of certain wavelengths, such as 2950 nm. Achieving a substantially dry coating is a known problem for sol-gel derived coatings.
Unlike typical antireflective coatings for visible light applications where the layer thickness is generally about 125 to 400 nm and can be formed as a single layer by the sol-gel method, antireflective coatings for infrared applications generally require substantially greater thicknesses, such as about 500 to 1,000 nm, and thus generally require a multilayer coating. Even a small absorption by individual layers of the multilayer coating, plus absorption from a second side identical to the first, can compound into large absorptions by the coating. The specific absorption at about 2950 nm due to water has traditionally been a problem for IR applications.
Infrared applications affected by the absorption at 2950 nm include thermal imagery and infrared positioning, regarding either spatial or distance acuity. In both of these systems multiple lenses are generally needed, and multiple lenses further degrade the transmission by increasing absorption. To avoid significant limitations to these devices, coatings that can transmit nearly 100% of the infrared light with very little reflectance are needed.
The typical technique used for preparing coatings that display little water absorption for these applications comprises Physical Vapor Deposition (PVD), typically a sputtering process where atoms of a vaporized gas are propelled towards the substrate, impinging and bonding to its surface. This technique is performed under vacuum and generally has a small amount of water present, but even this small amount of water requires removal by high temperatures (e.g. >150° C.) to further reduce the amount of water present. Aside from the significant cost and complexity introduced by performing PVD over the traditional wet chemical synthesis, PVD transfers significant energy into the substrate material by the energy transfer when the vapor is impinged on the surface of the substrate.
These energy sources as well as the thermal energy of the drying can adversely affect the substrate material, particularly for certain amorphous substrate materials (e.g. chalcogenide glasses). Semiconductor glasses, such as certain chalcogenide glasses, are a common substrate choices for certain advanced optical devices since they generally transmit across the full range of the infrared regime of the electromagnetic spectrum. For example, energy from the PVD process or high temperature processing (e.g. >150° C.) can undesirably cause some partial crystallization of the amorphous substrate material, resulting in scattering at the interfaces with the crystallites, and as a result, a lowered transmittance.
Thus, there is a need for a low temperature method for forming low water content thin film coatings on substrates and resulting substrates having low water content coatings thereon. For infrared optical coatings, the method should introduce little energy into the underlying substrate to avoid partial crystallization of the amorphous substrate or damage in the case of certain polymer substrates.